This invention relates to a perpendicular magnetic-recording medium and, more particularly, to a perpendicular magnetic disc and a method of manufacturing the same.
Recently, attention is drawn to high density perpendicular magnetic-recording effected by utilizing the residual magnetism retained in a plane perpendicular to the recording surface of a magnetic recording medium. A recording medium applied in said perpendicular magnetic-recording has hitherto been formed of a thin Co-Cr based layer prepared by sputtering. The reason is that said thin Co-Cr based layer has an axis of easy magnetization extending in a direction perpendicular to the plane of recording.
A hard disc for perpendicular magnetic-recording normally has its substrate prepared from an aluminum alloy. Aluminum alloy, however, is soft, so that its surface is anodized to provide a hard Alumite (trademark) layer. The surface of this Alumite layer is adapted for polishing, thereby providing an extremely smooth plane. But impurities contained in the aluminum alloy obstructs the stable formation of the Alumite layer. It was therefore necessary to rigidly control the purity of the aluminum alloy. A further problem was experienced in that even when the thin Co-Cr based layer was sputtered on the surface-anodized aluminum-alloy substrate, it was impossible to effect the C-axis dispersion adapted for perpendicular magnetization. In this connection, reference may be made to "Co-Cr Rigid Disc for Perpendicular Magnetic Recording", Sendai Symposium on Perpendicular Magnetic Recording (by Kazuo Kobayashi et al) Mar. 11 to 12, 1982. This publication describes improvement on the aforementioned C-axis dispersion by providing a titanium layer between the thin Co-Cr based layer and Alumite layer. However, the proposed improvement is still accompanied with the drawbacks that an increased internal stress arises, and the complexities of the method of manufacturing the subject Co-Cr based disc are experienced. Moreover, the thin Co-Cr based layer has to be prepared with a substrate maintained at a high temperature in order to increase a coercive force Hcv playing an important role in perpendicular magnetic recording. If, however, the thin Co-Cr based layer is formed with the substrate maintained at an extremely high temperature, cracks will appear in the Alumite layer due to a difference between the thermal expansions of the aluminum alloy and Alumite layer, depending on the thickness of said Alumite layer. As described above, many difficulties were encountered in the application of a surface-anodized aluminum alloy as a substrate of, for example, a Co-Cr based disc.